1. Field of the Invention
The invention generally relates to coating apparatus for use in gas or vapor deposition. The invention also generally relates to coating apparatus for use in semiconductor vapor doping. More specifically, a reactor design includes a multizone chamber to carry out a method of uniformly coating a stationary workpiece on a stationary work support.
2. Description of the Prior Art
In the art of growing semiconductor films, uniform growth is essential. A single crystal epitaxial film is often grown on a suitable substrate by the chemical vapor deposition (CVD) process, in which the elements of the desired crystal are introduced in gas phase compounds that are cracked by heat into their constituents in the area of the substrate, where the appropriate elements combine to form the crystal layer. A variety of factors, well-known in the art, tend to cause uneven crystal growth and variation in crystal composition over the surface of a substrate. For example, when a group 3-group 5 compound semiconductor such as GaAs.sub.1-x P.sub.x is grown with vertical gas flow against a horizontal substrate, the thickness of the crystal may be greater in the center of the substrate and thinner at the edges. At the same time, the percentage of gallium phosphide in the alloy may be lowest in the center of the substrate and greatest at the edges. The resulting crystal is subject to substantial differences in overall thickness, local composition, and local electrical characteristics.
Therefore, the use of chemical vapor deposition (CVD) processes involves special reactor design criteria. Two reactor configurations generally are used to satisfy the uniformity requirement. In the first, commonly used in a horizontally oriented reaction chamber, a pedestal supports the substrate to be coated at an acute angle to the impinging gas flow. The optimum substrate angle is empirically determined, involving considerable time and effort. In a second configuration, often used in a vertically oriented reaction chamber, the substrate is supported on a pedestal in a plane perpendicular to the direction of gas flow and, additionally, the pedestal is rotated in the perpendicular plane. A combination of these two methods has been used in vertical flow systems, wherein the substrate is carried at an acute angle to the input gas flow with the pedestal being rotated on an axis parallel and concentric to gas flow.
These known methods involve a variety of problems and disadvantages. One problem is that deposition uniformity is altered by convection currents. The substrate usually is carried on a susceptor of graphite, which is heated by RF heating to cause the desired cracking. Convection currents that result from the heating may alter the gas flow that otherwise would cause uniform crystal growth. Another problem arises from the use of a rotating pedestal, which requires a leak-proof rotating seal to avoid system contamination, in turn resulting in complicated and costly reactor designs. Another problem is that few reactors can meet the high standards for uniformity that are required for the growth of a "superlattice," involving the formation of multiple thin (i.e., 100 Angstrom) layers of differing composition on the same substrate. It is desired to grow such superlattices in a single, continuous process by changing the composition of the input gas at rapid intervals, which requires that the reactor be able to grow a uniform layer under all of the different gas compositions and with a sharp interface between changed compositions.
A more simplified approach to preparing uniform semiconductor films has been a long time objective by many skilled persons in the semiconductor film deposition art. It would be of particular advantage to eliminate the need for a rotating pedestal in vertical flow reactors. Similarly, it would be desirable to eliminate the need to empirically determine the optimum angle for the pedestal in horizontal flow reactors. Still another benefit would result by eliminating convection current problems. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the apparatus and method of this invention may comprise the following.